University of Heidelberg

Experimental Test Results and Further Developments

Distribution of Pulse Hights
The Detection Cascade
High-Rates Capability
Spatial Resolution
Actual Prototype

Detailed calculations with respect to detection efficiency and spatial resolution have been performed in preparation to the layout of solid converter coatings. A thorough understanding of the pulse hight distribution with varying converter thickness could be obtained. Further, various coating processes were investigated, and some quite stray, ingenious methods invented, in order to obtain stable neutron-converting coatings on the GEM-foils.

Pulse Hight Distribution

Single Boron coatings on GEMs allowed to confirm the theoretical models towards the pulse hight distribution.

Pulse hight distribution as measured on a 400 nm Boron coating.

Pulse hight distribution as measured on a boron coating of about 2.3 mm thickness.

The multi Boron layer cascade

The first operable coated GEM-foils (4cm x 4cm) allowed to prove unambiguously that cascaded GEM-foils do serve as charge-transparent substrate for thin neutron converting coatings such as Boron. Spatial resolution was better than 3 to 5mm and the spatial information could be imaged without distortions through several successive coated GEM-foils onto a signal readout electrode. The detector is operatad at ambient pressure and is absolutely insensitive to gamma background.

Spatial spectrum of a position sensitive CASCADE detector. The data was generated with a CASCADE prototype containing four cascaded Boron coatings and four GEMs. Individual GEMs were successively switched to charge-transparent mode. The different data sets correspond to the GEMs being switched transparent one by one, adding additional converter coatings. Each additional converter coating clearly gives stepwise additional detection efficiency.

Neutron Detection Rate Capability

TOF spectrum measured in the direct beam of PF1a at the ILL/Grenoble with a CASCADE-Prototype of size 100mm x 100mm. Instantaneous count rates of 2.7 MHz have been observed on one single readout channel without saturation of the detector.

The background level is due to the neutron gas scattered on air within the casematte of PF1a and due to a leakage in the inhomogeneous absorbing coating of the chopper disk (see the wiggle at 50 Angström which depends on chopper frequency).

A Linear plot of the TOF spectrum is given here.

Detection Efficiency

This image shows a plot of theoretically obtainable detection efficiencies for thermal neutrons (1.8 Angstrom) for the CASCADE-detector as a function of the single 10Boron layer thickness. The parameter given lables the number of twosidedly coated GEMs, or equivalently, half the number of 10Boron layers involved.

Data of detection efficiency measured on a three layer 10Boron-layer prototype at the SANS beamline at PSI/Switzerland, on a 8 layer 10Boron-layer prototype at the CT2 instrument at Ill/Grenoble compared to the corresponding theoretical prediction.
Predicted detection efficiency for 20 cascaded 10Boron-layers. Note 50% detection efficiency for thermal neutrons. Through a variation of individual layer thickness, detection efficiency can be customized to e.g. enhance efficiency on a certain band of wavelengths or, alternatively, obtain a "black" detector beyond about 5 Angstrom neutron wavelength.

Spatial Resolution

The image above shows data taken at FZ Jülich with a collimated neutron beam and a collimator orifice of 0.57mm in diameter. A single readout strip of the 2D-readout structure has 1.57mm width. These measurements were performed with a 200mm x 200mm CASCADE area detector and a mixture of counting gas 90/10 Argon/CO2. The precise value of spatial resolution can be manipulated through the choice of the counting gas mixture. Typical value for spatial resolution with ambient operating pressure is 2.6mm. For special configurations (several bar of overpressure), resolution can be as small as 1mm.

Actual Prototype

A CASCADE Prototype with a sensitive area of 200mm x 200mm.

IT Department